Quadra 800 (Wombat) overclocking

[AlpineRaven]

Hey all,
As I have Quadra 800 logic board inside Quadra 700 case, it also has DOS compatible card installed with extension to the NuBus ports as well.
(Quadra 700 logicboard was battery cancer, and Quadra 800 case shattered into pieces hence two into one - Love the shape of Quadra 700 case, and it has feets too!

Anyway; I love Quadra 840av but I don't like moving it around too much as its so brittle, but if I could get Quadra 800 faster than 840av then I am very happy with that.

I have researched and I have found 3 options as I already have spare 68040 40mhz CPU;

Option A) Replace the Crystal Oscillator from 16.667mhz to 20.0 mhz and install 68040 40mhz CPU - then it'll run at 40 mhz.

Option B) Remove crystal oscillator and install Spicy O'Clock.

Option C) Install and run 68040 40mhz CPU, install jumper in J28 & 29 plus replace R152 resister to 1.2kΩ, replace to 330Ω in R233, and no resister in R151, URL link below;

Making a Wombat Fast

Making a Wombat Fast Or, how to bump your Centris 650, Quadra 650 or Quadra 800 to 40 MHz with minimal effort. A 40 MHz Quadra 650 (“Current System”) Traditionally, the way we have over…

elephantandchicken.co.uk

Thread 'New Awesome and Easy Way to Bump a Wombat to 40MHz'

Mar 11, 2024

I've found a new way to overclock Wombats.

If you have a 33MHz Q650 or a Q800...

Step 1 - bridge J29 with solder, it is next to the CPU.

That's it.

Your Wombat will now boot at 40MHz.



The machine photographed above has a 16.6MHz clock fitted at G3!

More detail.

So it turns out that the chip U71 is only there to select between two clock sources, all shipped Wombats, it is hard wired to route the clock output from G3, the 12.5 or 16.6MHz clock, to U21, which doubles it's frequency and sends it to the CPU... But it turns out that if you fit J29, it selects an alternative...

• Phipli
• Replies: 18
• Forum: Mac II, Quadra & Centris

I am not interested any faster than 40mhz, I am thinking lending towards option B that way I can switch over from 40mhz to 33mhz if I wanted to.

Lastly; I am running System 7.1, my other concern which I haven't found info for - will it still boot in 7.1 - if so which enabler should I use?
Cheers
AP

 

[zigzagjoe]

I recommend phipli's method, option C on your list.

If the machine was originally 33mhz then there is not a need to change the gestalt jumpers unless you want to use the 40mhz timings. Otherwise, it will run at 40mhz with 33mhz timings - an overclock, but IME a safe one on any wombat board. There is no need to change the CPU as your machine will have come with a E42K or E31F mask chip which will do 40mhz without issue as long as it has the heatsink on it and case is closed.

Takeaway is: Put a small switch at J29, next to the CPU. On = 40mhz, off = 33mhz. Nothing else is needed. Easy.

 

[ObeyDaleks]

I'm really curious why, according to the linked article, the overclocked Quadra 800 video performance is so much higher than the 840av. Does anybody know the technical reason behind this?

 

[Phipli]

I'm really curious why, according to the linked article, the overclocked Quadra 800 video performance is so much higher than the 840av. Does anybody know the technical reason behind this?

Because the ROM is still set up assuming 33MHz timings.

If you set it to 40MHz timings it goes back to a more normal video speed (but still faster than 33MHz performance). If you use 33MHz ROM settings at 40MHz you're actually overclocking, rather than if you use 40MHz timings (with a 40MHz CPU) you're just... Getting a higher end computer.

The down side is your serial ports and floppy drive are less likely to work? Possibly? And potentially reliability issues (but mine runs fine like that).

 

[Phipli]

the overclocked Quadra 800 video performance

Also, that is an overclocked Centris 650 (it started life as a 25MHz 68040 powered machine), not that there is a real difference

That li'l Centris will push quite far
Here she is doing 50MHz, with video performance exceeding a 7100 HPV card! She's even stable like this. Ignore the "Centris 839" that's just me editing the computer's name in the System file.

 

[ObeyDaleks]

Because the ROM is still set up assuming 33MHz timings.

If you set it to 40MHz timings it goes back to a more normal video speed (but still faster than 33MHz performance). If you use 33MHz ROM settings at 40MHz you're actually overclocking, rather than if you use 40MHz timings (with a 40MHz CPU) you're just... Getting a higher end computer.

The down side is your serial ports and floppy drive are less likely to work? Possibly? And potentially reliability issues (but mine runs fine like that).

Thanks for the explanation. I didn't realize that the combination of higher CPU clock and lower timings actually provides the best performance, while higher timings provide better stability. Makes perfect sense now (although maybe not intuitively obvious).

 

[ObeyDaleks]

Also, that is an overclocked Centris 650 (it started life as a 25MHz 68040 powered machine), not that there is a real difference

That li'l Centris will push quite far
Here she is doing 50MHz, with video performance exceeding a 7100 HPV card! She's even stable like this. Ignore the "Centris 839" that's just me editing the computer's name in the System file.

Oh, I didn't realize that was your article. That's a mighty Centris 650 you have there!

 

[Phipli]

Oh, I didn't realize that was your article. That's a mighty Centris 650 you have there!

Yeah it's my first / favourite Wombat. I only found out about the jumper for taking 20MHz from the SCSI clock after I'd also fitted a socket for replacing the main clock, so mine actually switches between 40MHz and 50MHz depending on if you fit the jumper or not